The present invention relates to lubrication oil systems for turbomachinery, such as steam turbines, and particularly relates to a secondary lubrication means utilizing a positive displacement pump.
Large turbomachines as, for example, large steam turbines, generally include a rotor shaft which is positioned and supported by a plurality of bearings axially spaced along the shaft. It is generally recognized that these bearings are flood lubricated with oil and the rotating shaft is supported by a film of oil present between the rotating shaft and the surface of the bearing. Commonly, the shaft is oriented horizontally with its weight supported by several journal bearings and its axial portion is maintained by a thrust bearing. It is well recognized in the art, that oil must be supplied to these bearings during substantially all operating phases of the turbine to maintain the integrity of the bearing surfaces in proximity to the shaft and to maintain the close clearances and tolerances between the turbine rotor and the stationary components of the turbine. The loss of a flow of oil to these bearings during any type of operation results in unacceptable consequences.
Several lubrication systems are currently utilized to supply oil to the bearings of large steam turbines. One prior art system utilizes a main shaft-driven pump as the primary source of pumping power in the lubrication system. This main pump is mechanically coupled to the rotor shaft and, hence, is driven by the shaft. This pump is normally a centrifugal pump which provides a sufficient flow of oil at a high pressure into its discharge line. Some turbines utilize a mechanical hydraulic control system for the control of the turbine. This mechanical hydraulic control system requires a high transient flow of oil to operate the control valves admitting motive steam to the turbine under transient conditions. A centrifugal main pump is capable of providing this high transient oil flow without substantial reduction in the head or oil pressure in the discharge line.
With the main shaft-driven pump located at the elevation of the centerline of the turbine, some means must be provided to supply oil from the oil reservoir at a lower level to the inlet of the main pump at sufficient pressure to satisfy suction pressure requirements of the main pump.
In one prior art lubrication oil system, oil at a high pressure in the main pump discharge line is utilized as motive fluid for a booster turbine/booster pump combination disposed in the oil reservoir. The booster pump has an input port in the oil reservoir and pumps oil into the main pump suction line which is fluidly connected to the input port of the main shaft-driven pump. High-pressure oil from the main oil pump outlet is fed into the booster turbine which mechanically drives the booster oil pump. In providing energy to drive the booster turbine a reduction in pressure occurs and therefore the oil exiting the booster turbine is at a lower pressure and is piped into a supply line which is fluidly connected to the bearing header and ultimately to the plurality of bearings. This prior art system is detailed fully in U.S. Pat. No. 2,440,980, Sheppard, the disclosure of which is incorporated herein by reference thereto.
The centrifugal main pump pumps oil through the lubrication system efficiently and reliably at turbine rated speed, without utilizing any other power source other than energy from the rotating shaft. However, a centrifugal main pump with an oil turbine-driven booster pump does not function when the rotational speed of the shaft drops below a predetermined value, normally about two-thirds of the design operating, or rated, speed of the turbine. As the speed of the main oil pump decreases, its discharge pressure decreases and, therefore, the speed and discharge pressure of the booster pump decreases. With the main oil pump speed below approximately two-thirds of design speed the discharge pressure of the booster pump is insufficient to overcome the increase in elevation in the line to the main pump inlet. As is recognized in the art, without sufficient suction pressure the main pump cavitates and its discharge pressure decreases even further, causing complete collapse of the system and cessation of oil flow. The adverse consequences of loss of lubricating oil flow to the bearings is well known in the art.
In further accord with the prior art lubrication system at least two additional centrifugal pumps whose outputs are fluidly connected to the supply line to the bearings of the turbine are used. One of these centrifugal pumps is driven by an ac motor and is used to supply oil to the bearings whenever the turbine is operating below the self-sustaining speed of the main oil pump/booster pump combination such as when the turbine is being started or shut down.
A control system including a pressure sensor in the lubricating oil supply line activates this ac motor-driven centrifugal pump when the pressure in the supply line falls below a predetermined point. Unfortunately, experience has shown that sometimes this ac motor-driven centrifugal pump does not begin pumping oil in time to maintain a sufficient flow of oil to the bearings. This may be because as the speed of the electric generator driven by the turbine and providing ac power for station auxiliaries decreases, a delay occurs in transferring to a source of ac power from outside the station. Also failure of such equipment as fuses and breakers can result in failure of the ac motor-driven pump to start.
The second centrifugal pump in the mentioned prior art is also fluidly connected to the lubrication oil supply line and is driven by a dc motor. The dc power source for such motor is normally a plurality of batteries on site at the power plant. A control system, similar in nature to that for the ac motor and centrifugal pump described above, actuates the dc motor centrifugal pump sub-system. However, field experience has shown that this dc motor-driven centrifugal pump does not always provide sufficient flow to the bearings of the turbine due to failure of the control system, intervention by the human operators of the turbine plant into the control system, failure of the dc power source, or interruption of the dc power source due to other occurrences at the turbine plant.